Background. Consistent with NIDA's and NIH's HIV priorities, this early career award aims to elucidate behavioral and social mechanisms associated with substance use and optimize treatment success among men who have sex with men (MSM) who are sub-optimally engaged in care. Internalized stigma and related emotions, such as shame, are under-addressed modifiable barriers to engagement in HIV care across the cascade, which theory indicates may mechanistically occur via avoidance coping behaviors, including substance use. As stigma-related emotions are notoriously under-reported, improved measurement and assessment of these pathways are needed to more strategically intervene on these barriers to HIV care. Research. Using two pre-existing datasets (n=313), I will assess the interrelationships between HIV care cascade indicators (i.e., linkage and engagement in care), substance use, and stigma-related emotions, informed by three measurements (recently validated nonverbal behavioral coding, narrative coding, and self- report). I will also assess whether avoidant coping mediates these relationships. I will then conduct focus groups with this target population, and use the obtained feedback with the results from the secondary data analysis to inform the refinement of a previously developed emotion regulation intervention (PI Batchelder) designed to improve engagement in HIV-care among substance using HIV+ MSM sub-optimally engaged in care. I will then conduct a pilot randomized controlled trial (RCT) of the refined intervention with substance using HIV+ MSM sub-optimally engaged in HIV care. Outcome data from this pilot will be used to assess the clinical utility for improving engagement in HIV care and reducing substance use. Training. To address gaps in my training, I will develop expertise in 1) content and methods to better understand the interrelationships between the HIV care cascade, substance use, and internalized stigma and related emotions, 2) targeted advanced statistics, and 3) developing and conducting clinical trials with substance using HIV+ MSM sub- optimally engaged in care. Mentorship. Providing complementary expertise central to my research and training goals, Dr. Conall O'Cleirigh is an expert in HIV-related intervention development and implementation involving MSM and Dr. John Kelly is an expert in substance use treatment interventions. Both have mentored K awardees that have successfully transitioned to independent investigators. The advisory team will provide additional expertise related to the training goals including the medical treatment of HIV, affective science, stigma, latent variable modeling and mediation, advanced RCT statistics including EMA, and text interventions. Candidate. I have been productive as a graduate student and postdoctoral fellow, with over 20 publications (4 additional manuscripts under review). I have received two pilot grants as principal investigator that enabled me to develop and conduct an open pilot of the aforementioned intervention. This K23 would address targeted gaps in my training that are crucial to my research agenda and development as an independent investigator.